A major cause of spinal stenosis or entrapment of neural tissues is the presence of hyperostoses, bone spurs, disc bars, osteophytes, and other excrescences from bony or bony/ligamentous overgrowth. The usual surgical procedure for relieving the consequent compression of the neural tissues and attendant blood vessels is to cut off the excrescence with an osteotome or a high speed air drill. All bony structures have two principle layers: an outer hard layer called the cortex and a softer, spongy inner layer called the cancellous portion. If the cortical portion is cut away in order to eliminate an excrescence, this may weaken the bone, possibly promote an overgrowth of the cancellous portion and hence a return of the excrescence, or may leave an irregular surface against which the nerve must lie. Furthermore, bone is covered with membranes and often with ligaments which when cut away in the usual fashion (air drills and osteotomes) may leave behind ragged edges of the adherent soft tissues. This may promote fibrosis (scar tissue formation).
Among the most common locations for compression of nerve and blood vessels in the human spine is the area that lies adjacent to an oval structure known as the pedicle. Major nerves coming out of the spinal cord and its containing sac pass around and beneath the pedicle almost like a rope passing around a pulley. Overgrowth of a portion of the pedicle may produce nerve compression which may be relieved by cutting away the overgrowth, but the cutting of this overgrowth or of any other excrescence may be difficult and lead to complications. For example, upon cutting away a bone spur which is a mixture of calcific and soft tissue along the margins of a disc bar, the remaining surfaces may be quite irregular or torn, and this may lead to redevelopment of spurs or to epidural or perineural fibrosis. Furthermore, the neural tissue is usually in such close proximity to the spur that removing it presents a mechanical hazard.